1. Field of the Invention
The present invention relates to a process for preparing a metal powder and particularly to a process for preparing a metal powder useful for a thick film paste.
2. Description of the Prior Art
In the field of electronics, thick film pastes, such as conductor pastes and resistor pastes, have been used for the production of electronic circuits and components, such as resistors, capacitors, and IC packages. The thick film paste is prepared by homogeneously mixing and dispersing a conductive powder, such as a metal, an alloy, or a metal oxide, optionally together with a vitreous binder or other additive(s), in an organic vehicle to prepare a paste. The paste, after application onto a substrate, is baked at a high temperature or heat cured at a relatively low temperature to form a conductor film or a resistor film.
The metal powder or alloy powder used in the above thick film paste is required to have the following properties.
(1) Good dispersibility, in a paste, enough to form a dense, uniform film.
(2) Low content of impurities having adverse effect on electric characteristics.
(3) Sufficient crystallinity to offer a proper sinterability.
(4) Particles having a particle size of from about 0.1 to 10 .mu.m and a uniform shape.
Conventional processes for preparing the above metal powder include a spray pyrolysis method described in Japanese Patent Publication No. 31522/1988, Japanese Patent Laid-Open Nos. 172802/1994 and 279816/1994 and the like. According to this method, a solution containing at least one metal salt is atomized to form droplets which are then heated at a temperature above the decomposition temperature of the metal salt, preferably a temperature around the melting point of the metal or above to heat-decompose the metal salt, thereby depositing a metal or alloy powder.
According to the spray pyrolysis method, a metal or alloy powder can be easily prepared which, as compared with a metal or alloy powder prepared by wet reduction or the like, has better crystallinity, higher density and higher purity and possesses properties suitable for a thick film paste. The particle size of the metal powder can be controlled by properly setting the concentration of the metal salt, solvent, atomization and heating conditions, etc. Further, since the metal composition of the resultant particles is in agreement with that of the starting metal salt in the starting solution, making it easy to control the composition, which in turn renders the spray pyrolysis method suitable for the preparation of a multi-component powder.
In general, the spray pyrolysis method can provide a powder having very good dispersibility because metal particles are prepared in a vapor phase under such conditions that the particle concentration is relatively low. Exposure to a very high temperature in the course of formation of a metal powder, however, is likely to cause fusing among particles, sintering and aggregation depending upon various conditions such as remarkable increase in particle concentration or formation of the particle in a turbulent gas stream, even when the temperature is not above the melting point. Also, in the step of cooling the resultant powder, strong aggregation among particles often occurs in some handling methods. In many cases, it is difficult to break the aggregates into particles, leading to poor dispersibility in a paste. Further, fusing or aggregation of the powder causes adhesion or deposition in the production apparatus or requires classification of the aggregate, resulting in a reduction in yield of the product and, in addition, rendering continuous operation difficult. This phenomenon is significant and, hence, raises a problem, particularly in metals having a relatively low melting point, such as a silver powder or an alloy powder having a high silver content.